This invention relates to directional acoustic microphones.
As is well-known in electroacoustics, directional properties are usually imparted to microphones by subtracting the sound pressure at one point from the sound pressure at an adjacent point, ordinarily from 1/8" to 1" away. Consider a tube, 1" long, with a vibratile diaphragm in the middle. The motion of the ##STR1## diaphragm will be proportional to the difference in sound pressure between entries A and B. There will be a time difference of 0.077 milliseconds from entries A and B for sounds originating at the left or right due to the velocity of sound. This is the maximum time difference for this construction and, hence, maximum output. Sounds originating in the plane of the diaphragm will arrive at exactly the same time and will produce no output. There is, of course, an additional delay of 0.0385 milliseconds inside the microphone (from entry A or B to the diaphragm), but this delay is added to both sounds and disappears in the subtraction.
If we establish an axis through the center, call the right entry 180.degree. and the left 0.degree., then the polar pattern from this microphone will have a zero at 90.degree. and 270.degree. and will have equal and maximum sensitivity at 0.degree. and 180.degree.. This pattern is termed bidirectional, and occurs whenever the internal time delay is equal on both sides of the diaphragm. It is used when sounds from the side are to be rejected.
If a cloth screen is placed over entry B, the right half of the tube is converted to a low-pass acoustic filter. Below the cutoff frequency, this filter introduces an additional time delay. If the filter introduces a delay of 0.077 milliseconds, then sounds approaching from the right (180.degree. ) will experience the following delays in milliseconds as it proceeds to the diaphragm by two paths:
______________________________________ Entry A Entry B ______________________________________ .077 (tube length) .077 (filter) .0385 (1/2 tube) .0385 (1/2 tube) Total .1155 .1155 A-B (difference) = 0, and hence sounds from the right will produce no output. For sounds approaching from the left (.degree.): .0385 (1/2 tube) .077 (tube length) .077 (filter) .0385 (1/2 tube) Total .0385 .1925 A -B (difference) = .154 msec. For sounds approaching from 90.degree. or 270.degree.: .0385 (1/2 tube) .077 (filter) .0385 (1/2 tube) Total .0385 .1155 A-B difference = .077 msec. ______________________________________
This is 1/2 of the delay at 0.degree., and hence 1/2 the output.
This microphone will have a maximum output at 0.degree., 1/2 maximum at 90.degree. or 270.degree., and zero at 180.degree.. This polar pattern is called a cardioid and is produced when the external delay is equal to the difference in internal delay from each of the two entries to the diaphragm. This pattern is used when rejection of sounds from the back of the microphone is desired.
When an interfering sound is present at an angle between 90.degree. and 180.degree. (say 125.degree. ), a pattern called the supercardioid may be used, which has a null response at that angle. This pattern is produced when the internal difference of delay is equal to 0.577 times the external delay.
The foregoing examples show how the polar pattern may be changed by changing the delay of the filter in the rear (B) entry. This is the variable normally used to design a microphone polar pattern for a specific application and is an element of construction; i.e., not adjustable from the outside. Also, this explanation has not dealt with features which make the response to various frequencies of sound have uniformity.